Conventionally, pulse width modulation (PWM) control is used to control electric current for driving a multi-phase rotary electric machine. For example, if the multi-phase rotary electric machine is a three-phase motor, the electric current supplied to the three-phase motor is controlled by (a) comparing a voltage instruction signal regarding a voltage to be applied to respective winding wires of three phases with a PWM reference signal such as a chopping wave or the like, and (b) switching between ON and OFF of switching elements of an inverter based on the comparison. Generally, between the power supply side and the ground side of the inverter, a capacitor is provided to smooth the power supply voltage.
Regarding electric current of the capacitor, when electric current does not flow from a battery to a power supply side of the inverter, the electric current flows from the battery to the capacitor, and the capacitor is charged. On the other hand, when the electric current flows from the battery to the power supply side of the inverter, the electric current flows out from the capacitor to the inverter, and the capacitor discharges electricity. When the inverter is controlled by PWM control, charging and discharging of the capacitor is repeated during one cycle of PWM control, and the capacitor electric current pulsates. Such pulse current is designated as ripple electric current.
Further, when one inverter and a group of three winding wires corresponding to that inverter is designated as one system, the capacitor electric currents from two systems add up if charging and discharging due to PMW control by the inverters in the two systems occur at the same timing, thereby causing an increase of the ripple electric current.
The increase of the ripple electric current causes noise and heat generation of the capacitor. Further, as the applied voltage on the inverter changes, the controllability of the electric current by the inverter deteriorates. Therefore, a motor driving apparatus in the following patent document 1 smoothes a waveform of capacitor electric current, by phase-shifting PWM reference signals (i.e., a carrier signal in the patent document 1) of inverters in two systems to have different switch ON-OFF timings of switching elements, for reducing ripple electric current.
Further, a PWM amplifier in the following patent document 2 reduces ripple electric current when a PWM amplifier is used simultaneously in two systems (i.e., in two axes in the patent document 2), by shifting a neutral point voltage (i.e., an average of voltage instruction signals) to a high voltage side in one system and by shifting the neutral point voltage to a low voltage side in the other system, thereby bringing switching ON-OFF timings of the two systems to different timings.
(Patent document 1) JP 2001-197779A
(Patent document 2) JP 2007-306705A
However, even after phase-shifting the PWM reference signals from each other in two systems, or even after shifting the neutral point voltages of the voltage instruction signals from each other in two systems, a shifting range of switch ON-OFF timing of the switching elements is limited, because the voltage instruction signals of two systems have the same phase in both of the patent documents 1 and 2. For example, when the difference between the maximum value and the minimum value of the voltage instruction signal, that is, a doubled value of the amplitude, exceeds 50% of an allowable voltage output range that can be output, the discharge timings of the capacitors of the two systems cannot be completely shifted away from each other, thereby making it impossible to sufficiently reduce the ripple electric current.